1. Field of the Invention
The present invention relates to the control of a DC—DC converter. More specifically, the present invention relates to the control of an output voltage with the stop of operation and the change in a set output voltage in a DC—DC converter of a synchronous rectification system.
2. Description of Related Art
Portable type electronic equipment uses a battery as a power source. The electric power of the battery is discharged with time according to power consumption with the operation of the equipment. The output voltage of the battery is lowered. To maintain the voltage value of the power source of the equipment constant to the change in the battery voltage with time, the DC—DC converter makes the voltage of supplied power constant.
Electronic equipment may use a plurality of voltage sources having different voltage values. A DC—DC converter may be provided for each of the voltage sources. In this case, with the start and stop of the electronic equipment, it is important that start and stop be performed in suitable order in consideration of the start and stop sequence of the voltage sources. Unless the order of the start and stop sequence is suitable, a semiconductor device constructing the electronic equipment can cause a latchup phenomenon in which the state of applying a forward bias to a PN junction part is maintained to continuously flow an undesired electric current. Upon occurrence of the latchup phenomenon, in the case of limiting no electric currents, the semiconductor device can be burnt.
To avoid these disadvantages, when the DC—DC converter is stopped, regardless of a heavy or light load, electric power stored in the capacitance on the load side such as an output capacitor must be efficiently drawn out to rapidly lower an output voltage. The rapid lowering of the output voltage can prevent the power source from remaining at stop and from performing unexpected operation. The start operation can be performed in the state that there is no remaining voltage in the voltage source to suitably perform the start sequence of the voltage source.
At stop, to efficiently draw out electric power stored in the capacitance on the load side such as an output capacitor for rapidly lowering an output voltage, a capacitance discharge path such as a bleeder resistance may be provided, which is not preferable because the path constantly consumes the electric power. Accordingly, for instance, a technique for forming a capacitance discharge path as needed as disclosed in Japanese unexamined patent publication No. H9 (1997)-154275 is proposed.
In a DC—DC converter of a synchronous rectification system shown in FIG. 6, in the operation state, energy stored in a choke coil L121 by the conduction of a main transistor Tr121 is discharged to an output voltage side VO by the conduction of a transistor for synchronous rectification Tr122 in the non-conductive period of the main transistor Tr121. The conductive control of the transistor for synchronous rectification Tr122 is performed by a synchronous rectification control circuit 250.
At stop, an ON signal ON is brought to low level. At this time, when a DSCHG signal is at high level, an AND circuit AND102 also outputs high level. An OR circuit OR101 constantly outputs high level regardless of the synchronous rectification control circuit 250. The transistor for synchronous rectification Tr122 is constantly brought to the conductive state via a drive circuit 260. The main transistor Tr121 is non-conductive by low level of the ON signal ON. At the same time, the conduction of the transistor for synchronous rectification Tr122 short-circuits the output side VO to the ground potential to forcefully discharge the capacitance on the load side.